The present invention relates to a process for manufacturing multilayer pellets and a die equipment for manufacturing multilayer pellets. In more detail, the present invention relates to a process for manufacturing multilayer pellets served as a molding material for various plastic products and having a core-sheath structure comprising a plurality of molding materials and a die equipment used for manufacturing multilayer pellets with the extrusion molding technique.
The present invention also relates to masterbatch resin pellets with improved brittleness and a process for manufacturing thereof using a multilayer pellet.
The present invention further relates to a process for drying and manufacturing thermoplastic polyester resin with good drying acceleration using a multilayer pellet.
Pellets for plastic molding are obtained by heating and melting plastic materials previously blended and extrusion-molding the melted materials into strand forms, and then cutting the obtained plastic strands into small tablet forms, so-called pellets.
When various plastic products are molded, preparing the pellets as described above has many advantages. For example, blending and adjusting materials are not required every time when plastic products are molded; stable operation of plastic molding can be performed; stabilized product quality can be obtained; and raw materials can be easily supplied to the molding machine and other handling becomes easier.
Plastic pellets entirely molded from one plastic material are common. However, double-layer pellets or multilayer pellets comprising a plurality of plastic materials and a process for manufacturing such pellets have also been proposed.
JP-A-7-171828 (xe2x80x9cJP-A-xe2x80x9d indicates Japanese Unexamined Patent Publication) discloses multilayer pellets of a core-sheath structure in which a material with rich adhesiveness is used as the core and a crystalline polyolefin resin with less adhesiveness is used as the sheath in order to solve the problem of likeliness to cause blocking of pellets manufactured from materials with rich adhesiveness.
JP-A-59-81121 discloses a process for manufacturing pellets from an olefin-vinyl alcohol copolymer which is unable to form strands for manufacturing pellets due to its too small melt tension. In the process, multilayer pellets of a core-sheath structure is manufactured by cutting multilayer strands having a core with resin having small melt tension, i.e, the olefin-vinyl alcohol copolymer, outer circumference of which is covered with resin having large melt tension. JP-A-59-81121 also shows a manufacturing equipment having a dice-head for pellet molding equipped with a structure for feeding sheath material to the circumference of core material.
With the conventional multilayer pellet manufacturing technique, the molding speed of multilayer strands must be set to a comparatively low level in order to successively cover the sheath material around the outer circumference of the core material when the multilayer strand is molded.
Consequently, the productivity of the multilayer strand and multilayer pellets is poor and manufacturing cost of multilayer pellets is high.
In order to solve this problem, a method of using a strand molding machine equipped with a plurality of dice heads, that is, extrusion ports, and manufacturing a plurality of multilayer strands simultaneously could be proposed.
However, it is difficult to adjust in such a manner as to successfully cover the outer circumference of the core material with the sheath material of a specified thickness at each of a plurality of extrusion ports. It is also difficult to feed the molten resin materials to a plurality of extrusion ports in a uniform quantity and at uniform flow rate and to control uniformly the flow of core material and sheath material throughout the full circumference of the flow.
One object of the present invention is to solve the above-mentioned problem and to provide a process for manufacturing multilayer pellets of a high quality and having a stable core-sheath structure at a high productivity.
As mentioned above, the present invention also relates to masterbatch resin pellets with improved brittleness and a process for manufacturing thereof using a multilayer pellet.
As methods for blending pigments; low molecular weight polymer such as wax, additives such as plasticizer, antioxidant, ultraviolet absorbent, light stabilizer, etc. (hereinafter called the xe2x80x9cpigments, etc.xe2x80x9d) in a thermoplastic resin such as polystyrene, polyester and polypropylene, a method for adding these pigments, etc. at the time of polymerization reaction; a method for adding masterbatch, in which pigments, etc. are dispersed in or stuck to a polymer at a high concentration, to natural resin; a method for adding, directly to polymer, the pigments, etc. or processed pigments or the like which comprises the pigments, etc. and dispersant such as low-molecular weight polyethylene or a metallic soap; and other methods are known. Of these, the method for adding masterbatch is most popularly used in various fields because of its handiness at the time of addition and excellent performance, for example, non-contamination, suitability to automatic weighing, and suitability to transportation.
The masterbatch comprises base resin as a main component, to which pigments, etc. are mixed, and in general, the masterbatch is used by diluting 2 to 150 times by a resin, so-called natural resin, at the time of its application. In general, for the base resin of the masterbatch, a resin identical to or similar to the natural resin is used because of the relation in properties to the natural resin or fluidity of the masterbatch. For the masterbatch manufacturing method, it is common practice to extrude the melt containing the base resin and pigments, etc. into a form of strand, and then, to cool the strand, and to cut it into pellets.
Since the masterbatch is diluted with natural resin at the time of application, it is desirable to contain pigments, etc. at a high concentration. However, manufacturing such a masterbatch containing a high concentration of pigments by a conventional method, strand breakage at the time of melting-extrusion and generation of chips at the time of cutting are caused due to the brittleness of the composition. In addition, since compatibility of the additives with the base resin is generally low, if a masterbatch containing a high concentration of additives is manufactured by a conventional method, the additives tend to be localized and the portions where the additives-localized becomes brittle, also causing problems of strand breakage at the time of melting-extrusion and generation of chips at the time of cutting.
Another object of the present invention is to provide a process for manufacturing master-batch resin pellets containing a high concentration of pigments, etc. free of strand breakage at the time of melting-extrusion and chip generation at the time of cutting. It is also an object of the present invention to provide masterbatch resin pellets obtained by the process.
The inventors have found that these objects of the present invention can be accomplished by using specific multilayer pellets manufactured by a process such as those mentioned above.
As mentioned above, the present invention further relates to a process for drying and manufacturing thermoplastic polyester resin with good drying acceleration using a multilayer pellet.
Thermoplastic polyester resin such as polyethylene terephthalate, etc. is composed with repetitive units having ester bonds. However, when it is heated in the presence of moisture, esteric bonds are hydrolyzed to change resin quality, and to cause problems of poor moldability, etc. Consequently, before molding, the moisture in the thermoplastic polyester resin must be removed, and from the industrial viewpoint, it is generally preferable to dry at sufficiently higher temperature than the glass transition point. If the resin is highly crystalline like polyethylene terephthalate or polybutylene terephthalate, no particular problem occurred even when drying was carried out at higher temperature than the glass transition point.
However, in recent years, cases, in which an amorphous copolyester resin or a polyester resin of low-melting point which scarcely possesses crystallinity have to be used, have increased in various applications such as paper containers for liquid, flavor-barrier sheet of snack packages, cap materials for APET, easy-peel material, extrusion wire coating, heat seal material, PET film modifier, softener, coil coating, shrink film, aroma retention heat seal film, binder fiber, composite fiber, heat-shrinkable film, adhesives, gas-barrier bottle, etc. When such resins are dried at temperatures above the glass transition point, they become fluidized, stick to the inside of drying container or the pellets adhere one another, causing blocking.
For solving the above problem, JP-A-9-164526 discloses a drying method of amorphous copolyester resin in which a crystalline polyester powder is mixed with an amorphous copolyester resin pellet and they are dried, preferably, in the agitated. However, since, in this method, the amorphous copolyester resin to be dried must be in a pellet-form, this method is unable to be applied for drying a polyester resin difficult to be pelletized. In addition, in this method, only limited drying equipment is able to be used. Furthermore, a comparatively large amount of crystalline polyester powder is required with respect to the amorphous copolyester resin.
It is a further object of the present invention to provide a process for drying thermoplastic polyester resin including an amorphous copolyester resin, low-melting point polyester resin or the like without causing fusion and sticking to the drying container inside or mutual adhesion of pellets, at high temperature, in a short time, and easily, even when they are dried at high temperatures causing blocking.
The inventors have conducted intensive studies in order to attain the above object. As a result, the inventors have found that a multilayer pellet having a core-sheath structure, wherein the core contains a thermoplastic polyester resin including an amorphous copolyester resin, a low-melting-point polyester resin or the like and the sheath contains a thermoplastic resin with specific properties against the core section, does not cause the said problem even when they are dried at high temperature that would cause blocking of the thermoplastic polyester resin used for the core. The pellet can be manufactured from a strand obtained by simultaneously extruding a thermoplastic polyester resin used for core and a thermoplastic resin used for sheath, and, in the strand, the thermoplastic polyester resin used for core is covered with a thermoplastic resin used for sheath and, if an appropriate resin is used, can be supported by a thermoplastic resin used for sheath. Therefore, even if the thermoplastic polyester resin used for core is a polyester resin difficult to be pelletized, it can be pelletized by support of the thermoplastic resin used for sheath. The present invention of a process for drying thermoplastic polyester resin was thus completed.
As the multilayer pellets, those manufactured by an above-mentioned process for manufacturing multilayer pellets of the present invention can be used.
The present invention provides a process for manufacturing multilayer pellets having a core-sheath structure with a molding material forming a sheath which covers around the outer circumference of another molding material forming a core, which comprises the following processes (a) through (c):
Process (a): supplying the core material and the sheath material to a die equipment having a plurality of extrusion-forming parts arranged along a circumference,
Process (b): extruding a plurality of multilayer strands from the extrusion-forming parts by covering around the outer circumference of the core material concentrically with the sheath material, and
Process (c): cutting the extruded multilayer strands to manufacture multilayer pellets.
The present invention also provides a master-batch pellet with improved brittleness, which comprises thermoplastic resin and at least one compound (A) selected from pigments and additives; and which has a core-sheath structure wherein the core layer contains the thermoplastic resin and the compound (A) and is covered with the sheath layer containing thermoplastic resin, and 80 wt % or more of the compound (A) are contained in the core layer.
The present invention also provides a process for manufacturing the said master-batch pellets, which comprises melt-extruding a thermoplastic resin and at least one compound (A) selected from pigments and the additives into a core-sheath strand so that 80 wt % or more of the compound (A) is contained in the core, and cutting the strand into pellets after the strand is cooled.
The present invention further provides a process for drying thermoplastic polyester resin at high temperatures causing blocking which comprises drying multilayer pellets having a core-sheath structure in which the thermoplastic polyester resin to be dried is used as the core and a thermoplastic resin which does not cause blocking at the said drying temperature is used as a sheath.
The thermoplastic polyester resin pellets of the present invention providing a process for drying thermoplastic polyester resin are characterized in that they do not cause blocking at drying temperature at which the said thermoplastic polyester resin to be dried itself causes blocking. Due to the characteristic, a high drying temperature, which causes blocking of the thermoplastic polyester resin to be dried, can be employed. As the result, drying can be accelerated and moisture in the thermoplastic polyester resin can be removed effectively in a short time.